Method of removing sulfur trioxide from combustion gases to reduce the corrosive effects thereof



Sept. 26, 1967 50 ox/o/zfp 70 50 200 300 700 560 600 700 800 900 /O00//00 lUO m 5 19/9. WEE C K URT FERDINAND INVENTOR W/CKE/F T ATTORNEYSUnited States Patent 3,343,908 METHOD OF REMOVENG SULFUR TRIOXIDE FROMCOMBUSTHGN GASES TO REDUCE THE CGRROSIVE EFFECTS THEREGF Kurt FerdinandWickert, Berlin-Siemensstadt, Germany, assignor to Deutsche GoldundSilber-Scheideanstalt vormals Roessler, Frankfurt am Main, Germany FiledFeb. 6, 1964, Ser. No. 342,953 Claims priority, application Germany,Feb. 13, 1963, 1) 40,856 4 Claims. (Cl. 232) ABSTRACT OF THE DISCLOSUREProcess for reducing dewpoint acid corrosion and fouling of surfacescontacted at temperatures between below about 400 C. and the ambientatmospheric temperature with combustion gases containing sulfur trioxideand water vapor comprising incoporating in a fine state of distributiona mixture of (1) finely divided basic material selected from the groupconsisting of calcium oxide, magnesium oxide, magnesium carbonate, basicmagnesium carbonate and mixtures thereof and (2) finely divided acidicoxide material selected from the group consisting of silicon dioxide,aluminum oxide and mixtures thereof, said finely divided basic materialand acidic oxide material having a surface area of at least 30 m. /g.,into such combustion gases when they have cooled to a temperature ofabout 400 C., the molar proportion of said acidic material being atleast 0.5 mol per mol of basic material calculated as oxide.

The present invention relates to an improved process for hindering aciddewpoint corrosion, particularly those caused by sulfuric acid, andfouling of equipment subjected to combustion gases in the lowertemperature region.

It is known that liquid and solid fuels, such as oil and coal, containsulfur which in the combustion gases is partially oxidized to sulfurtrioxide. Such sulfur trioxide forms sulfuric acid vapor at temperaturesaround 250 C. with the water vapor which is always also present in thecombustion gases and such sulfuric acid vapors, depending upon theirconcentration in the combustion gases, deposit at 180 C. and lower uponthe available surfaces of the equipment in contact with such combustiongases. The strong corrosive effects of the deposition of the sulfuricacid under some circumstances can even be noticeable in the chimneysprovided for exhausting the cooled combustion gases to the atmosphere.In addition, the sulfuric acid which may be carried along with theexhausted combustion gases under unfavorable weather conditions cancause considerable pollution of the atmosphere which can be disturbingto health, as well as plant growth, in the surrounding territory.Attempts have already been made to bind the sulfur trioxide contained inthe exhaust gases with magnesium oxide or magnesium compounds such asdolomite and thereby prevent any occurrence of an acid dewpoint.However, only unsatisfactory results have been attained thereby as itwas only possible to shift the acid dewpoint to somewhat lowertemperatures. Previously, attempts to eliminate the more or less strongfouling caused by deposits of solids have not been successful to anydegree worth mentioning. To the contrary. the deposits of the substancesadded to reduce the dewpoint which have partially been converted tosulfates produce hard firmly adhering crusts, for example, in the lowtemperature region of the boiler tubes and the plates of the airpreheaters, which cannot be removed with the usual means withoutinterrupting the operation.

3,343,908 Patented Sept. 26, 196? According to the invention it wasfound that the occurrence of an acid dewpoint and the deposit of firmlyadherent encrustations on the surfaces contacted with combustion gasesin the lower temperature region could be practically completely avoidedif a mixture of highly disperse finely divided basic and acidic oxidesis uniformly distributed in the combustion gases when they have cooleddown to about 400 C. Instead of basic oxides, carbonates or basiccarbonates of elements forming such oxides can be used. Highly dispersesilicon dioxide or aluminum oxide or mixtures thereof have provedespecially suited as the oxides of acidic character which are usedaccording to the invention and magnesium oxide or calcium oxide ormixtures thereof are especially suited as the basic oxides. Magnesiumcarbonate or basic magnesium carbonate may also be used with advantagein place of the magnesium oxide or calcium oxide. It is essential forthe success of the process according to the invention that the highlydisperse acidic and basic materials employed are of sufficient finegrain that their surface area measured by the BET method is at least In./g. Preferably, the surface area of the acidic oxides is larger, forexample, about 100 m. g. The primary particle size of the acidic oxide,especially of the silicon dioxide, should be under 150 mu and preferablybe under 100 m Under these conditions, practically all of the sulfurtrioxide is bound in a very short period of time to magnesium sulfate orcalcium sulfate, for example, in less than 1 second. Magnesium oxide isespecially suited as the basic component of the oxide mixture as itpractically does not react with sulfur dioxide.

The quantities of the oxides employed insofar as the basic component isconcerned depends upon the quantity of sulfur trioxide present in thecombustion gases to be treated. The quantity of such basic oxide shouldbe stoichiometric with respect to sulfate formation. In order to insureprevention of the formation of encrustations and solid deposits at least0.5 mol of acidic oxide should be employed per mol of alkaline oxide.

The addition of the oxide mixture to the combustion gases as indicatedabove should be at temperatures not substantially above 400 C. asfurther formation of S0 by the catalytic oxidation of S0 practicallydoes not occur under this temperature.

The accompanying drawing is a graph showing the equilibrium curves 0nthe catalytic oxidiation of S0 to S0 plotted against the temperature.

In such graph, curve a is the normal equilibrium curve, whereas curves[2 and 0 give the amounts of the S0 oxidiation with deposited oil ash band with deposited coal fly ash c as the catalyst. Curve d correspondsto the conversion of S0 to S0 in a quartz apparatus. As can be seen fromthese curves, if the addition of the oxides to the combustion gases ismade when the latter still have a temperature of 600 C., further S0 canbe formed after the neutralization of the S0 present at such temperaturewith the stoichiometric quantity of the basic oxide such as magnesiumoxide and as a consequence the further cooled gases will still con tainS0 as the basic oxide was all constuned in the neutralization of the S0present at 600 C.

Evidently the effect of the acidic oxide in the oxide mixture employedaccording to the invention is that the finely divided oxide with largesurface area prevents the formation of firmly adhering encrustationswhich would have been produced from the magnesium sulfate by the actionof the water vapor contained in the combustion gases throughinterstratification.

The introduction of the oxide mixture into the combustion gases can beeffected at a suitable location at which the combustion gases havecooled to about 400 C. by blowing such mixture in solid form as a dust.However, according to an advantageous embodiment of the invention, suchoxide mixture is introduced into the combustion gases in the form of anaqueous dispersion. For this purpose the disperse oxide mixture, isfirst dispersed in water and such dispersion blown into the combustiongases with the aid of an injector or other suitable device. The aqueousdispersing agent vaporizes quickly at the 400 C. temperature, leaving auniformly distributed oxide smoke which effectively binds the su lfurtrioxide chemically and through the presence of the acidic oxideeffectively prevents the formation of encrustations.

In view of the fine particle size and large surface area of the oxidesemployed according to the invention, they are easily converted to stableaqueous dispersion. In the system of magnesium oxide and water, a solidsconcentration of about 8% by weight should be selected whereas in thesystem of silicon dioxide and water it can even be about 20% by weight.A 1:1 mixture of silicon dioxide and magnesium oxide can still beprocessed to a suitable dispersion at a 10% by weight solids content.

Example A boiler of a power station needs 10 t./h. of fuel oil which abelow heating value of 9,600 kcaL/kg. The excess air is 10%. The oil hasa sulfur content of 3.0% in the average.

1 kg. of fuel oil generates at burning with 10% of air excess a smokegas volume of 12.4 Nmfi. 10 t of fuel oil generate under the sameconditions a waste gas amount of 124,000 Nm. This waste gas contains 600kg. of S if the Whole sulfur would be burned to S0 The waste gasexamination showed that the thintieth part of this S0 amount consists ofS0 that means that 20 kg. of S0 were transformed into 25 kg. of S0 These25 kg. of S0 will be neutralized by blowing in of MgO. Theoretically12.5 kg. of MgO will be needed for this. It has been practically Workedwith an MgO excess of On a spot where the smoke gas stream had atemperature of about 400, a mixture of 14.0 kg. of MgO and kg. of SiOwas blown in per hour. By the treatment of the waste gas according tothe invention the content of kg. of S0 in 124,000 Nm. of Waste gas(corresponding to 20 kg. of 80;) was reduced to 0.37 kg. of 50;; or0.0003 g. SO /Nm. of waste gas. This S0 concentration will no moreeffect a raising of the water vapor 'dewpoint, that means an aciddewpoint is not anymore demonstrable.

The Water vapor dewpoint of a Waste gas containing i 10 vol-percentwater vapor is at a temperature of 40- 45 C. It raises up to 150180 C.,if there is a small content of S0 in such gas-mixture.

SiO was obtained by the hydrolysis in the gas phase in the pyrogenicmanner, the primary particle size was about 0005-0025 11, the specificsurface according to the BET method 190 m. /g. The MgO had a specificsurface of about -m. g. and consisted of scaly agglomerates up to 3 1size.

Analogical results will be obtained by replacing of silicon dioxide inthe whole or partly by titanium oxide and magnesium oxide in the Wholeor pantly by calcium oxide or basic or neutral magnesium carbonate.

I claim:

1. A process for reducing dewpoint acid corrosion and fouling ofsurfaces contacted at temperatures between below about 400 C. and theambient atmospheric temperature with combustion gases containing sulfurtrioxide and Water vapor comprising incorporating in a fine state ofdistribution a mixture of (1) finely divided basic material selectedfrom the group consisting of calcium oxide, magnesium oxide, magnesiumcarbonate, basic magnesium carbonate and mixtures thereof and (2) finelydivided acidic oxide material selected from the group consisting ofsilicon dioxide, aluminum oxide and mixtures thereof, said finelydivided basic and acidic oxide materials having a surface area of atleast 30 In. /g., into such combustion gases when they have cooled to atemperature of about 400 C., the molar proportion of said acidicmaterial being at least 0.5 mol per mol of basic material calculated asoxide.

2. The process of claim 1 in which said basic material is finely dividedmagnesium oxide having a surface area of about 30 mF/g. and said acidicoxide is silicon dioxide having a surface area of about m. g.

3. The process of claim 1 in lVVhlCh. the quantity of basic materialincorporated in the combustion gases is substantially stoichiometric forthe formation of the corresponding sulfate with the sulfur trioxidecontained in such combustion gases.

4. The process of claim 1 in which said mixture of finely divided basicmaterial and finely divided acidic oxide material is supplied to thecombustion gases in the form of an aqueous dispersion.

References Cited UNITED STATES PATENTS 2,718,453 9/1955 Beckman EARL C.THOMAS, Primary Examiner.

1. A PROCESS FOR REDUCING DEWPOINT ACID CORROSION AND FOULING OFSURFACES CONTACTED AT TEMPERATURES BETWEEN BELOW ABOUT 400*C. AND THEAMBIENT ATMOSPHERIC TEMPERATURE WITH COMBUSTION GASES CONTAINING SULFURTRIOXIDE AND WATER VAPOR COMPRISING INCORPORATING IN A FINE STATE OFDISTRIBUTION A MIXTURE OF (1) FINELY DIVIDED BASIC MATERIAL SELECTEDFROM THE GROUP CONSISTING OF CALCIUM OXIDE, MAGNESIUM OXIDE, MAGNESIUMCARBONATE, BASIC MAGNESIUM CARBONATE AND MIXTURES THEREOF AND